If the gas appears to be coming from one of the carburetors, the likely cause is the float level needs to be reset. If the gas appears to be coming from under the tank, then look for a bad gasket at the cut-off valve or on the tubing from the cut-off valbe to the carbs. These are the most likely causes. If these are not the source of the problem, then pin-hole leaks in the tank are a likely possibility, but this is more difficult to solve - a tank sealer may do it or may not seal it.